Drying cellular organic products



Jan. 25, 1944. B. R. WARNER 2,339,825

DRYING CELLULAR ORGANIC PRODUCTS Filed May 28, 1942 lvl/ah j.

Patented Jan. 25, 1944 Benjamin R. Warner, Evanston, Ill., assigner toTheGuai-dite Corporation Application May 28, 1942, Serial No. 444,909 4claims. (c1. 34-15) This invention relates to a method of dryingcellular organic products and more particularly to a method ofcontrolling the drying of such products in an atmosphere of steam atpressures below atmospheric in such a way as to produce the maximum rateof drying and to produce a product which rehydrates with the maximumspeed and eiiectiveness.

The invention has particular reference to processes such as thosedescribed in the copending application of applicant and John M. Baer,Serial No. 428,506.

In this connection I have discovered the very surprising fact that themore rapidly a given product is dehydrated under a predetermined set ofconditions, without altering its physical state, the more rapidly andefficiently it reabsorbs water, and the farther its drying can becarried without destroying its ability to rehydrate.

Ability to rehydrate may be reduced or destroyed by a great variety offactors, some of which are known and some of which are not. For example,it is well known that products can be dried to such an extent that theywill not readily pick up water, and in some cases will reacquire it onlywith extreme difficulty, if at all. Sometimes this change is due to aprofound chemical alteration in the product. In other cases it appearsto be due merely to a collapse o r elogging of the capillaries.

It is obvious of course that under any given set of conditions the rateof drying may be increased by raising the available heat input, but, ifthis is carried on too far, the ability of the material to rehydra-temay readily be destroyed by scorching or burning. It is, therefore,important not to dry at too great a rate, and for the reasons alreadygiven, it is essential to dry at as rapid a rate as is possible.

I have discovered that the rate of drying may be mathematicallydetermined for any given Inaterial, apparatus, and source of heat, andthereafter the process may be regulated in accordance with suchdetermination. Control is effected according to the following formula:

Zin: rate of water removal.

Ap--the diierence in vapor pressure between the vapor pressure of waterat the temperature of the product and the vapor pressure of water in thesurrounding atmosphere.

w=the weight of water which has been removed from the product. wa=theweight of original water content of the product. =the natural logarithmbase. k1 and k: are constants.

The constants k1 and k: may be determined from the experimental datacontained in any trial runs. They must be determined of course for anygiven apparatus. The constants are relatively close for various types ofmaterials if the state of subdivision is substantially the same and theinitial water content is substantially the same. The formula may besubstantially simplified and the need for the determination of theconstants eliminated if desired by plotting log p against w. It may beshown mathematically that when the resulting curve is a straight line.

. Q 1s a maximum.

It appears that when the product is dried at less than the maximum rate,the cell walls and particularly the capillaries may shrink to such apoint that rehydration will not occur, whereas the use of the maximumrate of drying keeps the cell walls and capillaries from contracting toa point in the drying far beyond that possible at slower rates.

A break in the curve 1 dw log A-p -dT against toward the end of thedrying indicates a basic change in the structure of the product, whichchanges the diffusion constant ka. Drying at the maximum ratesubstantially delays this break inthecurvesothatitispossibletodrytheproduct to a greater degree withoutaltering its `basic structure.

The control has been found suitable on drying vegetables, such ascarrots, beans, potatoes and sweet potatoes. and meat products ofvarious types. The products preferably are dried in small particles.

Thetemperature of the product may be obtained by the insertion oithermocouples within it. In making this determination it is advisable toemploy a considerable number of thermocouples and use the averagethereof as the temperature of the product.

The i'orm of apparatus or the exact type of drying is not part of thisinvention. Any suitable vacuum chamber may be employed and variousheating means may be utilized. Preferably the heat energy necessary issupplied predominantly by radiant heat but this is in the preierred formsupplemented by the use of superheated steam. It is highly desirablethat substantially all air be eliminated from the system. If it is noteliminated, the determination of Ap is extremely diillcult.

It is obvious that the particular units employed in measuring thevarious factors above described are unimportant. This is particularlytrue in determining the maximum rate of drying inasmuch as the selectionof units merely changes the slope of the curve, and the slope isgenerally unimportant. In some cases it is desirable to select unitswhich will give a curve having a slope in the neighborhood of 45 ratherthan one which is either close to or close to 90.

The equation given in claims 1 and 2 may be solved by application ofwell-known mathematical and engineering principles to a test run.

The table below gives the data of a test run in which the product wasl/" cubes -oi' diced meat This equationmaythenbewtintothetorm l dw 108 pa; loud-bww.

5 bTlrmtaking the log of each side of the equation.

a-'h lo is minus which had been precooked. The weight of the meat was1800 grams and the original weight of water content (wn) was 1040 grams.'I'he opera It is obvious that the curve represented by the aboveequation is a straight line. Furthermore, the slope of the curve is k:since the equation is of the i'orm y=b|m:r. In an equation of the formll=b+mx, m is the slope of the curve and in the above equation Ica isthe equivalent to the m in the form equation.

Similarly loge k1 equals the y intercept since if we equals zero, theloge ki represents the only value on the right hand side of theequation.

Figure 1 in the drawing hereof shows the plotting o1' the aforesaidcurve from which it is obvious that k: is approximately 7.7 and ki isapproximately 33.2.

Furthermore, since the values lie within experimental error of astraight line the drying is being conducted at the proper rate.

In carrying out the above work, any suitable form of drying apparatusmay be used and the instruments required are simply thermometers,

pressure gages' and scales. The weight of the water removed may bemeasured by condensing the steam in the exhaust and measuring the amountof super heated steam supplied to the system. An apparatus suitable rorperforming the operation is illustrated in the co-pending application ofapplicant and John M. Baer ingpressurewas57 m. m. 428,506, filed January28, 1942.

vlpol NSS 'r u" am; in d mp m P ein w 1 dw 1 a Timeinmin. oi totem .ofm.m. L Log.,

(yr.) wayin omg. mm wv mm' r d "'11 p m. m. of 5 mercury (p) 101 ma asce com se las 2.94 0.54 109 64. 0 7.0 145 0.139 79 11.8 2.42 0.85 11270.0 13.0 235 0.226 90 6.93 1.04 L11 116 76. 0 2l. 0 325 0. 312 90 5.53 1. 7l 1. 32 122 93.0 26.0 398 0.382 73 2.8 1.03 1.415 126 104 34. 0450 0. 442 62 1. 82 0. 598 1. 53 128 109 52. 0 521 0. 501 6l 1. 17 0.157 l. 72 134 126 69. 0 570 0. 547 49 0` 71 0. 342 1. 84 141 151 94. 0623 0. 602 56 0. 596 0. lill 1. 97 148 180. 133 682 0.655 56 0.421 0.8682.12 160 248 191 729 0. 700 47 0. 246 l. 385 2. 28 185 445 388 780 0.750 5i 0. 131 2. 04 2. 59 m9 700 543 826 0. 794 46 0. 0715 2. 659 2. 81212 76o vos ses assi 4o n.051 asis ass 212 16o 'me 884 assi es co4 amass 216 780 723 910 0. 880 32 0. D41 3. 219 2. 86 .21e 18o m um 0.895 25cosa a4 ass 216 780 721 954 0. 917 23 0. 029 3. 5 2. 86 21s vso m wesolo 25 0.029 as aaa In applying th results of this table in order tocalculate k1 and ka the formula is ilrst reduced to a form in whichthese may most readily be calculated. This may be done by dividing bothsides of the equation by p which gives the following equation:

The same form of apparatus and the same type of measurements may beemployed in making the calculations called for in the other claims.I In

fact, the formula given in claims 3 and 4 is a simpliiled method ofcalculating the same results shown by claims 1 and 2.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom.

What I claim as new, and desire to secure by Leters Patent, is:

l. The method of drying a, cellular organic product naturally containinga substantial quantity of water which comprises regulating the rate ofdrying in accordance with the formula E :klApekW/U:

where dw t= rate of Water removal,

2. The method of drying a cellular organic product naturally containinga substantial quantity of water which comprises regulating the rate ofdrying in accord-ance with the formula E15: klApel'lUn where =rate ofwater removal,

product.

Ap=the diiIerence in vapor pressure between the vapor pressure of waterat the temperature of the product and the vapor pressure of water in thesurrounding atmosphere,

w=the weight of water which has been removed from the product,

w=the weight of original water content of the product.

e=the natural logarithm base,

k1 and k2 are constants.

to produce substantiallycomplete drying without a signmcant change inthe structure of the product;

3. The method of drying cellular organic water-containing material whichcomprises regulating the rate of drying to produce a straight line curvewhen plotting ApT against Ap, where Ap represents the diierence betweenthe vapor pressure of water at the temperature of the product and thevapor pressure of water in the atmosphere surrounding the product, and Tequals the time o: treatment.

4. The method of drying cellular organic water-containing material whichcomprises controlling the rate of drying to produce a straight linecurve in plotting the log Ap against w. where Ap=the dilerence in vaporpressure between the vapor pressure of water at the temperature of theproduct and the vapor pressure of water in the surrounding atmosphere.and w=the weight of water which has been removed from the BENJANHN R.WARNER.

' l Certificate of Correction Patent No. 2,339,825.V January 25, 1944,

It is hereby certified that errors appear in the` printed speeicationpfthe above numbered fpatent requiring correction as follows: Page l,first column, hne 50, m the o A I.

- equation, r

A' u n gli read {2t-w* page 2, in the-ama sixth column from the left,aan une from boum; fr".o.s 31" read 0.850; and that the said LettersPatent should be read with these corrections there' that the same mayconform to the record of the case in the Patent Oice.

Signed and sealed this 14th day of March, A. D. 1944.

LESLIE m ding ojfPatmtr.

